Pre-stretched film having improved edges

ABSTRACT

Rolls of pre-stretched cling film are used to manually wrap and unitize bulky loads such as boxes on a pallet. Using film that has previously been plastically stretched simplifies the wrapping operation. Unfortunately, however, the pre-stretched rolls of film have edges which are prone to fail during use. The disclosed invention concerns rolls of pre-stretched cling wrap having improved edges that are less prone to fail, and the method and apparatus for producing such rolls.

FIELD OF THE INVENTION

[0001] The current invention concerns a method, apparatus, and product comprising a pre-stretched thermoplastic film having improved edges.

BACKGROUND OF THE INVENTION

[0002] The use of thermoplastic stretch or cling films has become a commonplace method of securing bulky load such as pallets filled with boxes. Generally, there are two types of film and wrapping methods, one referred to as machine wrap, the other as hand wrap. Both types of these films are typically made largely from the various polyethylene resins, and maybe made as single or multilayer products. Frequently an additive known as a tackifier or cling agent, e.g. polyisobutylene, will also be added to ensure that adjacent layers of film will adhere to each other.

[0003] Machine wrap films are typically used in powered pallet wrap machines, e.g. see U.S. Pat. No. 5,040,356 for an example. On the other hand, hand wrap utilizes an operator to stretch the film manually around the load. As the efficient use of the wrap requires that it be plastically stretched around the load, various devices, e.g. U.S. Pat. Nos. 4,600,163 and 4,752,045, have been devised to help the operator in this task. More recently, however, various methods of pre-stretching the film have been devised, e.g. U.S. Pat. No. 5,458,841 and WO89/06594, which will simplify application of the wrap to the load. With these pre-stretched hand wrap films, the operator merely wraps the film around the load without plastically stretching the film. As the elastic memory of the film recovers, the film contracts about the load.

[0004] One problem, however, with these pre-stretched hand wraps is that the edges of the film are subject to damage resulting in tearing of the film during use. Typically, the edges of the pre-stretched hand wrap film have been prepared by transversely slitting individual roll widths of film from a wider width by means of a conventional sharp edge slitter assembly. The film edges so prepared are anything but uniform, and will contain numerous small defects such as rips and tears. These rips and tears, plus the relatively limited ductility remaining in the heavily pre-stretched material leads to failure during application. One method of reinforcing the edges of the pre-stretched film has been devised, folding the edges of the material to form a hem, e.g. see U.S. Pat. Nos. 5,565,222 and 5,531,393. These hems, however, cause difficulties winding a uniform roll of film due to the essentially doubled thickness of the hemmed edge as compared to the remainder of the film. U.S. Pat. No. 5,531,393 discloses axially oscillating the roll during winding to minimize piling up of the hemmed edges on the roll.

OBJECTS OF THE INVENTION

[0005] It is an object of the present invention to provide a novel roll of pre-stretched hand wrap film having edges resistant to mechanical damage that results in tearing of the film during application to the load. Additionally, the film will have edges of uniform thickness that will simplify winding of uniform rolls.

[0006] A further object of the present invention is to provide a new method and apparatus for manufacturing pre-stretched hand wrap film having improved edges. A still further objective of the present invention is to maximize the useful width of pre-stretched hand wrap film manufactured by longitudinally slitting a greater width of stretch film into individual narrower rolls by avoiding the necessity of folding over or hemming the edges of the individual rolls to prevent tearing when applied to a load.

SUMMARY OF THE INVENTION

[0007] According to the present invention, a roll of pre-stretched hand wrap film having improved edges is produced by longitudinally slitting and annealing a previously stretched width of thermoplastic stretch wrap either by thermally slitting or by slitting and applying heat and then winding. By thermally slitting or slitting with the application of heat, the edges of the resulting narrower rolls are thermally annealed, resulting in edges of greater ductility and tear resistance than the remainder of the width of the film.

[0008] According to another aspect of the invention, there is a method for producing rolls of pre-stretched stretch wrap comprising the steps of directing a moving web of previously stretched film to a slitting means, slitting the web longitudinally to produce one or more individual webs having a narrower width dimension than the original web, and thermally annealing the slit edges of the web. In the preferred method, there will be an additional step of winding the slit and edge annealed web to produce the roll of improved wrap.

[0009] The apparatus comprises a guide means arranged largely transversely to the direction of motion of the previously stretched film to support the film and direct it into contact with a slitting blade and annealing means. Preferably, the guide means constitutes a guiding and support surface over which the film passes, said guiding and support surface having apertures into which a heated blade assembly may pass to contact, sever and anneal the film edges.

BRIEF DESCRIPTION OF THE DRAWINGS

[0010]FIG. 1 shows a roll of pre-stretched film, including a microscopic view of a slit edge of film that has been slit by a conventional sharp edged blade and wound, all in accordance with the prior art.

[0011]FIG. 2 shows a roll of film, including a microscopic view of a slit edge of film, that has been slit by a thermal slitting and annealing blade and then wound, all according to the present invention.

[0012]FIG. 3 is a plan view of a thermal slitting arrangement in accordance with the present invention.

[0013]FIG. 4 is an elevation view of a thermal slitting arrangement in accordance with the present invention.

[0014]FIG. 5 is a drawing of the thermal slitting and annealing blade used in the apparatus shown in FIGS. 3 and 4.

[0015]FIG. 6 is an elevation view of a thermal slitting and annealing blade according to another embodiment of the present invention.

[0016]FIG. 7 show a further embodiment of the present invention in which the slitting and annealing means comprise separate means.

[0017]FIG. 8 is a detail on the construction of the annealing means of FIG. 6.

DESCRIPTION OF THE PREFERRED EMBODIMENT

[0018]FIG. 1 shows a roll of film 10, including a microscopic view of a slit edge 13 and hemline 12 of a roll 10 of pre-stretched film produced according to prior art methods. As shown, the roll 10, which was wound on core 15, has tapered or conical ends 11 resulting from axial oscillation of the roll during winding. Additionally, the microscopic view shows the fold 12 for the hem, and the actual slit edge of the film 13. There are numerous defects 14 that can serve as initiation points for failure during use.

[0019]FIG. 2 shows a roll of pre-stretched film 55, including a microscopic view of an edge 20′, of pre-stretched film 20 produced by slitting and annealing with a heater blade in accordance with the present invention. As shown, the roll has remarkably square edges 56, as there was no need to axially oscillate the roll during winding to distribute hemmed material. Additionally, thermally slit and annealed edge 20′ is essentially uniform and defect fee.

[0020]FIG. 3. and FIG. 4 show the apparatus for producing an improved film roll in accordance with the present invention. A previously stretched film web 20 is directed into slitting and annealing station 30 over guiding means 25 such as conventional idler rolls, stretcher rolls or bowed rolls. Slit and annealed individual webs, 20 a through 20 e in FIG. 3 are discharged from the slitting and annealing station 30, over bowed roll 40 to winder 50 or winders 50 a through 50 e where the webs are wound into individual rolls. Additionally, narrow strips of material 20 z, which formed the original edges of web 20, are either discarded or recycled.

[0021] As shown, slitting and annealing station 30 comprises two main assemblies, a transversely oriented guide and support surface 31 and slitting and annealing knife assembly 33. Guide and support surface 31 preferably has a convex surface over which pre-stretched web 20 passes and one or more openings 32 in support surface 31. Additionally, there will be at least one heated slitting and annealing blade 33. One blade 33 is required for each longitudinal slit. As shown in FIG. 3 and FIG. 4, there are six heated slitting and annealing blades as pre-stretched web 20 is to be subdivided into five final webs, 20 a through 20 e and two edge trims 20 z. Active surface 35 of heated slitting and annealing blade 33 is brought into contact with web 20 and further recessed into openings 32 of support surface 31. As web 20 contacts and passes heated slitting and annealing blade 34, blade 34 will heat and cut through web 20 producing two edges, 20′ and 20″, as well as heat affected zones, also referred to as annealed zones 21′ and 21″ adjacent to the slit edge 20′ and 20″ respectively. As is well understood to those familiar with the web handling art, heated slitting and annealing blades 33 will be mounted to a support structure, preferably one that will permit blades 33 to be moved from an inactive position to an active position in contact with web 20. In the interest of clarity, however, the support structure is not shown in the Figures.

[0022] Also as shown in FIG. 3 and FIG. 4, a bowed roll or other web spreading device 40 is adjacent to the discharge side 39 of slitting and annealing station 30. Bowed roll 40 is used to separate the slit edges 20′ and 20″ of web 20 to ensure that slit edges 20′ and 20″ do not stick together in case of contact prior to complete cooling. Following slitting, the now individual webs 20 a through 20 e are directed to one or more winding machines 50. As will be understood to those familiar with winding technology, many different types of winding devices may be used. As shown in FIG. 3 and FIG. 4, however, individual staggered winders 50 a through 50 e are shown. With this arrangement, each individual slit and annealed web 20 a through 20 e will be wound on its own respective winder 50 a through 50 e.

[0023]FIG. 5 shows a closer view of slitting and annealing blade 33. As previously indicated, slitting and annealing blade has an active surface 35 that contacts web 20. Contact of web 20 with active surface 35 produces slit edges 20′ and 20″ as well as annealed zones 21′ and 21″ adjacent slit edges 20′ and 20″ respectively. Body portion 34 of blade 33 contains one or more cavities 36 containing electrical cartridge heaters 37 that are used to heat blade 33 to its operating temperature. Operating temperature will depend upon material thickness and the speed at which web 20 passes blade 33. Preferably blade 33 will also contain a temperature measuring device such as thermocouple 38, the output of which will be directed to a temperature control device, as is well known to those versed in the art.

[0024] An alternate type of heated slitting and annealing station 30′ is shown in FIG. 6. In this instance the transverse guide means 31 takes the form of an upstream idler roll 31 a and downstream idler roll 31 b. Web 20 passes over idler rolls 31 a and 31 b, and is contacted by heated slitting and annealing blade 33 in the space between rolls 31 a and 31 b. This blade is manufactured from an electrically resistive material such as Chromel-A, and is directly heated by passing electrical current through the blade. The blade has an active edge 35′ which will contact web 20, to produce slit edges 20′ and 20″ along with the associated annealed zones 21′ and 21″. Temperature of blade 33′ is controlled by varying the current flowing through the blade 33′. Current in turn is controlled by supplying power to blade 33′ from a variable supply such as a Variac.

[0025]FIG. 7 shows another configuration of a slitting and annealing device useful in the practice of the current invention. As shown, the device consists of a transverse web guiding and support device shown generally as 31″, a separate annealing device 70, and a slitting device 33″. In operation, web 20 passes over the first roll 31 a′ of the web guiding and support device 31″, where a narrow annealed zone 21 is produced in the web 20 by annealing device 70. Annealing device 70 preferably takes the form of an arc shaped shoe which closely conforms to the outside diameter of roll 31 a′ and which discharges heated air onto web 20. Alternately, annealing device 70 could be an arc shaped shoe closely conforming to the roll that is heated and which would heat web 20 by radiation or convection. Web 20 then encounters slitter blade 33′″ which forms a longitudinal separation 21 of web 20. Slitter blade 33′″ is aligned with annealing device 70 so that the longitudinal separation is centered within annealed band of web 20. Slitter blade 33′″ may be any device typically used for longitudinally slitting a web, including a sharpened blade 33′″ as shown in FIG. 7. Heated blades of the types shown in FIG. 3 and in FIG. 4 as 30 or if FIG. 5 as 30″ may also be used. After slitter 33′″, web 20 passes over a second roll 31 b″, and then to winders 50.

[0026]FIG. 8 shows the preferred form of annealing device 70. As shown in FIG. 8, annealing device 70 will have a hollow body 71 having an air inlet 72, a heating chamber 73, and a heated air distribution chamber 74, which will apply heated air to web 20. Heating chamber 73 will contain one or more cartridge heaters 75 to heat the flowing from inlet 72 to distribution chamber 74. As previously indicated, heated air distribution chamber 74 is arc shaped to conform closely to roll 31 a′. Additionally, distribution chamber 74 has a porous wall 76 that faces web 20. The actual discharge of heated air from the distribution chamber 74 to web 20 is through porous wall 76. Web 20 is thoroughly heated by annealing device 70, which anneals a narrow band 21 of web 20.

[0027] As can be seen in FIG. 3, the improved rolls of pre-stretched cling film of the present invention are produced as follows. The pre-stretched film web 20 is brought from a source of supply into the slitting and annealing unit over a series of rolls or other appropriate web guide devices. The source for web 20 may be a master roll of pre-stretched and wound film, or it may be the discharge from a pre-stretching unit such as a machine direction orientation device or a cold pre-stretcher of the types identified in U.S. Pat. No. 5,458,841 or WO89/06594. Within the slitting and annealing unit 30 the film is brought across the surface of the transverse film guide surface 31 and contacted by heated slitting and annealing knife 33.

[0028] Under the influence of the heated blade 33, the film will be separated longitudinally. Additionally, immediately adjacent to the longitudinal separation the web will have seen sufficient heat to anneal, i.e. reverse the effects of the prior stretching, the film. The optimum temperature at which to operate the heated slitting and annealing knife 33 will depend upon the material and thickness of web 20 and the speed at which it passes through the slitting and annealing unit 30. At the very least, however, the temperature of the slitting and annealing knife 33 must exceed the melting temperature of web 20. For the types of materials from which these pre-stretched cling films are prepared, melting temperature are frequently on the order of 120° C.

[0029] The now longitudinally slit web 20 leaves the transverse guide surface 31 over discharge bowed roll 40. Bowed roll 40 is used to separate the slit edges 20′and 20″ to prevent the possibility of these edges sticking together in case of contact with each other while sill tacky. After the bowed roll the individual webs 20 a through 20 e are led over a series of nip rolls and idler rolls to one or more winders 50. As is well known to those involved with the web handling and winding arts, there are many different ways in which the separate webs may be wound. As shown in FIG. 3 and FIG. 4, however, each web goes to a separate winder. Other possibilities include all webs going to individual cores on a single winder shaft, or alternate webs being wound on cores on alternate shafts, a process referred to as offset winding.

[0030] It will be understood by those generally familiar with handling plastic webs that the particular embodiments of the invention disclosed in this document are illustrative and are in no way meant to limit the invention. Numerous changes and modifications may be made and the full use of equivalents made without departing from the spirit or scope of the invention as defined in the following claims. 

I claim:
 1. A process for producing a roll of pre-stretched film have improved edges comprising; supplying a pre-stretched web of thermoplastic material to slitting and annealing station, slitting the pre-stretched web, thermally annealing the web along the slit line, and winding the slit and annealed material into rolls.
 2. A process according the claim 1 wherein the slitting and annealing occurs by contacting the pre-stretched web with a heated slitting and annealing blade, whereby slitting and annealing are accomplished simultaneously.
 3. A process according to claim 1 wherein the annealing step occurs independently of the slitting step and the annealing step comprises heating a narrow strip of web by means of a heating device.
 4. A process according to claim 3, wherein the heating device is adapted to conform closely to the surface of the pre-stretched web.
 5. A process according to claim 4 wherein the heating device is arc-shaped and adapted to closely conform to the surface of the web as it passes over a roll.
 6. A process according to claim 3 wherein slitting is accomplished by means of a heated blade.
 7. Apparatus for slitting and edge annealing a pre-stretched web of plastic film comprising: inlet guide means for directing a web of pre-stretched plastic into a slitting and annealing station, support means for said plastic web in said slitting and annealing station, slitting means for slitting said plastic web and producing a slit edge, annealing means for annealing said web adjacent said slit edge, and discharge guide means for directing said slit and edge annealed web away from said slitting and annealing station.
 8. Apparatus according to claim 7 wherein said slitting means and said annealing means constitute a thermally heated blade means that contacts said plastic web.
 9. Apparatus according to claim 8 wherein said support means comprises and arc-shaped shoe adapted to receive said slitting means and said annealing means.
 10. Apparatus according to claim 7 wherein said annealing means comprises a narrow heating device adapted to closely conform to the web.
 11. Apparatus according to claim 10 wherein said heating device is arc shaped and adapted to closely conform to the web as is passes over a roller.
 12. Apparatus according to claim 11 wherein said heating device heats and anneals by applying hot air to the web.
 13. Apparatus according to claim 7 further comprising a winder for forming a roll of edge slit and annealed web.
 14. A roll of pre-stretched film having slit and annealed edges.
 15. A roll of film according to claim 14 wherein the film comprises a layer of material containing low density polyethylene or linear low density polyethylene.
 16. A roll of film according to claim 15 wherein said roll of film is wound on a core. 